1. Field of the Invention
The present invention relates to a method and a device for the transmission of data between at least two users of a bus system, in which the time duration of the bits transmitted is able to be switched over between at least two different values.
2. Description of the Related Art
From published German patent application document DE 100 00 305 A1, for example, the Controller Area Network as well as a “Time-Triggered CAN” (TTCAN)-designated broadening of the CAN is known. The media access control method used in the CAN is based on a bit-wise arbitration. In a bit-wise arbitration, a plurality of user stations are simultaneously able to transmit data via the channel of the bus system, without thereby interfering with the data transmission. Furthermore, the user stations are able to ascertain the logical state (0 or 1) of the channel while transmitting a bit over the channel. If a value of the bit sent does not correspond to the ascertained logical state of the channel, the user station terminates the access to the channel. In CAN, the bit-wise arbitration is usually carried out in an arbitration field within a data frame that is to be transmitted via the channel. After a user station has sent the arbitration field completely to the channel, it knows that it has exclusive access to the channel. Consequently, the end of the transmission of the arbitration field corresponds to the beginning of an enable interval, within which the user station is able to use the channel exclusively. According to the protocol specification of the CAN, other user stations may not access the channel, that is, send data to the channel, until the sending user station has transmitted a checksum field (CRC field) of the data frame. Thus, an end point of the transmission of the CRC field corresponds to the end of the enable interval.
Because of the bit-wise arbitration, a non-destructive transmission of the data frame over the channel is achieved. Because of this, good real time properties of the CAN come about, whereas media access control methods, in which the data frame sent by a user station is able to be destroyed during transmission via the channel, based on a collision with a further data frame sent by another station, have a clearly more unfavorable real time behavior since, based on the collision and the new transmission of the data frame required thereby, a delay in the data transmission comes about.
The protocols of the CAN are particularly suitable for transmitting short messages under real time conditions. If larger data blocks are to be transmitted via the CAN domain, the relatively low bit rate of the channel becomes a limiting factor. In order to assure the correct functioning of the bit-wise arbitration, during the arbitration, for the transmission of a bit, a minimum duration, that is first of all a function of the extension of the bus system, the signal propagation speed on the channel and intrinsic processing times in the interface modules of the bus users, has to be maintained, because all the bus users have to have a uniform picture of the state of the bus (0 or 1) and equal priority access to the state of the bus. Therefore, the bit rate cannot be increased without any problem by reducing the duration of the individual bits.
So as, nevertheless, to be able to transmit sufficiently rapidly a relatively large data block, required for the programming of a control unit, via a communications interface that is actually provided for connection to a CAN domain, German patent application document DE 101 53 085 A1 proposes switching over the communications interface for transmitting the data block temporarily to another communications mode, in which no bit-wise arbitration is carried out, and consequently a relatively high bit rate is possible. In this case, however, the communication with the protocols of the CAN has to be interrupted for a certain time. If, for example, based on an error, the operation of the bus system cannot be taken up again according to the CAN protocols, there is a failure of the bus system. In addition, the transmission of a relatively large data block will cause a considerable delay in the transmissions to be undertaken according to the protocols of the CAN, so that the real time properties of the CAN are impaired.
Published German patent application document DE 103 11 395 A1 describes a system in which the asynchronous, serial communication is able to take place alternatively via an asymmetrical physical protocol or via the symmetrical physical CAN protocol, and because of this, a higher data transmission rate or data transmission security is achievable for the asynchronous communication.
Published German patent application document DE 10 2007 051 657 A1 suggests using an asynchronous, rapid data transmission not conforming to the CAN in the exclusive time windows of the TTCAN protocols, so as to increase the transmitted data quantity.
G. Cena and A. Valenzano, in “Overclocking of controller area networks” (Electronics Letters, vol. 35, No. 22 (1999), p. 1924) treat, from a theoretical point of view, the effects of overclocking the bus frequency in subsections of the data frame on the effectively achieved data rate, without, however, going into details of the methodology and the various states and state transitions of the bus users.
It may be seen in the cited documents that the related art does not supply satisfactory results from every point of view.